The present invention relates in general to a waveguide filter, and more particularly, to a ridge waveguide filter having a slow-wave structure.
Waveguide filters have been widely known to provide outstanding performance at microwave frequencies compared to other technologies such as microstrips, striplines or even coax transmission lines. Depending on the configurations and dimensions, low-pass, high-pass, and band-pass waveguide filters have been developed to separate the various frequency components of a complex wave. FIG. 1 shows a conventional rectangular waveguide. The rectangular waveguide is typically a hollow metallic tube with a rectangular cross-section. According to IRE standards, the coordinate system as shown in FIG. 1 includes the x direction taken as the longer transverse dimension, the y direction taken as the shorter transverse dimension, and the z direction taken as the longitudinal dimension. The conducting walls of the waveguide confine electromagnetic fields and thereby guide the electromagnetic wave. As known in the art, the rectangular waveguide is normally very bulky and costly. Although the lately developed micro-machine technique seems to resolve the cost issue, the dimension of the rectangular waveguide is still too large to be useful.
To resolve the size issue, ridge waveguides have been proposed by introducing single ridge or multiple ridges into the rectangular waveguides. The introduction of a ridge loads the waveguide with a shunt capacitance and therefore reduces the characteristic impedance of the waveguide. As a consequence, the cross-sectional area required for operation at a certain frequency is reduced compared to the rectangular waveguide, but the decreased impedance leads to two deleterious effects, including increased loss (degraded performance) due to the increased current that must flow through the conductive walls, and the limited bandwidth obtainable in coupling structures connecting to the ridge waveguide.
George Goussetis discloses a periodically loaded E-plane filter in IEEE Microwave and wireless components letters, Vol. 13, No. 6, June 2003. The E-plane filter is formed by loading periodically reactive obstacles in form of ridges in a conventional rectangular waveguide. Such E-plane filters, though providing a slow-wave structure, does not resolve the cross-sectional size issue of the rectangular waveguides, and do not take advantage of the increased impedance.
Therefore, there is a substantial need to provide a waveguide filter structure that includes a slow-wave structure and has a reduced size. Further, the characteristic impedance of such a waveguide filter will not be reduced because of size reduction.